The subject invention relates generally to methods and apparatuses for calcining gypsum, and in particular to methods and apparatuses for continuously calcining natural gypsum, synthetic gypsum, or combinations of natural and synthetic gypsum. Calcined gypsum, more commonly known as stucco, is useful as a major ingredient of gypsum wallboard and plaster-based products. Stucco has the valuable property of being chemically reactive with water and will “set” rather quickly when the two are mixed together. It is this quick setting time that makes stucco ideal to work with in the mass production of wallboard.
Typically, wallboard consists essentially of a gypsum core sandwiched between two sheets of paper and is used as a cost-effective replacement of conventional plaster walls. To be commercially profitable, wallboard is typically manufactured by continuous high speed processes. Typically, gypsum (calcium sulfate dihydrate) predominately makes up the wallboard. Manufacturers mine (or receive synthetic gypsum) and transport gypsum to a board mill in order to dry it, grind it and calcine it to yield stucco (the “milling process”). Drying refers to the removal of the free water from the gypsum (water not bonded to calcium sulfate) and calcination refers to the conversion of calcium sulfate dihydrate to calcium sulfate hemihydrate. The general reaction for the creation of stucco is characterized by the following equation:CaSO4.2H2O+heat→CaSO4.½H2O+1½H2O
This equation shows that calcium sulfate dihydrate plus heat yields calcium sulfate hemihydrate (stucco) plus water vapor. This process is normally conducted in a flash calcination impact mill, of which there are several types known in the art. One such example is an impact mill simultaneously dries, grinds, and calcines the gypsum to produce stucco.
More specifically, the flash calcination of raw gypsum results in several gypsum phases as described by the following reactions:CaSO4.2H2O+heat→CaSO4 (AIII)+2H2O (Soluble Anhydrite)  I.CaSO4.2H2O+heat→CaSO4.½H2O+1½H2O (Hemihydrate)  II.CaSO4.2H2O+heat→CaSO4 (AII)+2H2O (Insoluble Anhydrite)  III.
As mentioned above, calcined gypsum (stucco) has the valuable property of being chemically reactive with water, and will “set” rather quickly when the two are mixed together. This setting reaction reverses the above-described stucco chemical reaction performed during the calcination step. The general reaction proceeds according to the following equation:CaSO4.½H2O+1½H2O→CaSO4.2H2O+heat
In this reaction, the calcium sulfate hemihydrate is rehydrated to its dihydrate state over a fairly short period of time. The actual time required for this setting reaction generally depends upon the type of calciner employed, reagents added, and the type of gypsum rock that is used. More specifically, the gypsum hydration reaction proceeds according to the following conversion reactions:CaSO4 (AIII)+2H2O→CaSO4.2H20+heat (Soluble Anhydrite Conversion)  I.CaSO4.½H2O+1½H2O→CaSO4.2H2O+heat (Hemihydrate Conversion)  II.CaSO4 (AII)+2H2O→CaSO4.2H2O+heat (Insoluble Anhydrite Conversion)  III.
It is, in part, the aim of the present invention to pretreat the resultant gypsum phases created by flash calcination, particularly the highly reactive and water demanding soluble anhydrite (AIII) phase, prior to the stucco's introduction in the pin mixer that form the basis for this invention.
In manufacturing wallboard, a “stucco slurry” is formed by mixing together dry and wet ingredients in a pin mixer. The dry ingredients can include, but are not limited to, any combination of calcium sulfate hemihydrate (stucco), fiberglass, accelerator(s), and in some cases natural polymer (i.e., starch) The wet ingredients can be made up of many components, including but not limited to, a mixture of water, paper, pulp and potash. The stucco slurry is then discharged from the mixer through a tube which spreads the slurry on a moving, continuous bottom facing material (e.g., cover paper), which is slightly wider than the desired board width. A moving, continuous top facing material (e.g., cover paper) is placed on the slurry and the bottom facing material so that the slurry is positioned in between the top and bottom layers of the facing materials to form a “wet wallboard.” Typically, forming plates are used to form the wallboard to the desired thickness and width. The board then travels along rollers for several minutes, during which time the setting reaction occurs and the board stiffens. The boards are then cut into a desired length and then fed into a large, continuous oven/kiln for drying. The end product is a wallboard with a gypsum core.
While conventional gypsum wallboard products have many advantages, it has also long been desired to reduce the cost of manufacturing gypsum wallboard. One method of reducing the cost of manufacturing gypsum wallboard has been to reduce the amount of water used in the manufacturing of the wallboard. Reduction in water reduces the amount of free water left in the wallboard after the setting reaction. A lower amount of free water left in the wallboard results in less drying energy being expended to remove the free water, which in turn saves energy costs associated with drying wallboard (i.e., the fuel cost associated with operating a kiln to dry the wallboard).